The use of nitrogen has become increasingly important in various industrial and commercial operations. For example, liquid nitrogen is used to freeze food, in the cryogenic recycling of tires, as a source of gaseous nitrogen for inerting, etc. Gaseous nitrogen is used in applications such as secondary oil and gas recoveries and as a blanketing gas in metal refineries, metal working operations and chemical processes, etc. In light of the increasing importance of nitrogen in such operations, it is desirable to provide a process which is both economical and efficient for producing nitrogen in the liquid and/or gas phase.
High purity gaseous nitrogen is produced directly by well-known cryogenic separation methods. U.S. Pat. No. 4,222,756 teaches a process and apparatus for producing gaseous nitrogen using multiple distillation columns and associated heat exchangers. Ruhemann and Limb, I. Chem. E. Symposium Series No. 79, pp 320 (1983) advocate a preference for the use of a single distillation column instead of the typical double column for the production of gaseous nitrogen.
Liquid nitrogen is typically produced by initially producing gaseous nitrogen in a cryogenic air separation unit and subsequently treating the gaseous nitrogen in a liquefier. Modified forms of cryogenic air separation units have been developed to directly produce liquid nitrogen. U.S. Pat. No. 4,152,130 discloses a method of producing liquid oxygen and/or liquid nitrogen. This method comprises providing a substantially dry and substantially carbon dioxide free air stream, cryogenically treating the air stream to liquefy a portion of the air stream, and subsequently feeding the air stream into a fractionation column to separate the nitrogen and oxygen, and withdrawing liquid oxygen and/or nitrogen from said column.
The production of medium purity nitrogen or oxygen by noncryogenic air separation processes is achieved by using absorption, adsorption and membrane based processes. U.S. Pat. No. 4,230,463 discloses multicomponent membranes which are effective for separating at least one gas from gaseous mixtures by permeation wherein the multicomponent membranes are comprised of a coating in occluding contact with a porous separation membrane. Selective separation provides preferential depletion or concentration of one or more desired gases in the mixture with respect to at least one other gas in the mixture, thereby producing a product gas mixture having a different proportion of gases than the original mixture. Membranes which provide an adequately selective separation of the one or more desired gases and also exhibit a sufficiently high flux rate are preferred.
Membrane units have been used in conjunction with cryogenic separation units for the production of gaseous oxygen. In these systems, air is initially fed to a membrane where a nitrogen-rich stream is produced and subsequently vented to the atmosphere. The oxygen-rich permeate stream is fed to the cryogenic system to produce a purified gaseous oxygen stream. Examples of this type of process are described in Japanese Pat. Nos. 58-156173 and 58-151305.